North Korea fired several short-range projectiles Thursday in what seemed to be an act of defiance against the expanded sanctions that were unanimously adopted by the United Nations Security Council.

South Korea’s Defense Ministry said it is investigating the incident. It is unclear exactly how many projectiles were fired or if they were short-range missiles, but the ministry said they flew 100 to 150 kilometers before falling into the East Sea, also known as the Sea of Japan.

The United States "will continue to monitor the situation closely," a U.S. official said.

A provocation of this type was not unexpected or unprecedented. North Korea has a history of voicing its disagreement with international reprimands through a show of force rather than through traditional diplomatic channels.

Focus on China

The new U.N. resolution punishing North Korea for its latest nuclear test and long-range missile launch had more than 50 co-sponsors, but was primarily negotiated between the United States and China.

China is North Korea’s closest ally and largest trading partner. Nearly 90 percent of all North Korean trade goes through China.

The effectiveness of the sanctions will depend in large measure on Beijing’s implementation and enforcement.

“As the agreement came from this [close collaboration] I think China's responsibility to keep its promise has been strengthened,” said Professor Kim Han-kwon with the Korea National Diplomatic Academy, which is affiliated with South Korea’s Foreign Ministry.

One measure calls for the mandatory inspection of all cargo going in and out of North Korea.

Crew members are seen on the 6,700-tonne freighter Mu Du Bong in the port of Tuxpan, April 9, 2015.

China will have to increase inspections at seaports such as Dalian and in the border city of Dandong, through which most trade passes.

There are also many unofficial transits points along the 400-km (870-mile) Sino/North Korean border where uncontrolled trade and commerce has grown in recent years.

Other measures include:

· A total arms embargo, including both conventional and other weapons.

· Increased financial restrictions on companies that do business with North Korea.

· Expanded prohibitions on luxury goods to North Korea.

· Travel bans and asset freezes for 16 new North Korean officials.

· Restrictions on coal and other mineral exports from North Korea, except for "livelihood purposes."

There have been reports by South Korean media that China has already begun restricting some border trade, suspending currency transfers with North Korean banks and prohibiting North Korean vessels from entering Chinese ports.

However, Andrea Berger, a British analyst with the Royal United Services Institute for Defense and Security Studies, said these reports should be “treated with skepticism and should not be viewed as an early indication of China’s intentions regarding the resolution.”

Weak link

Writing for the U.S. Korea Institute website 38 North she suggested China will most likely continue to be the weak link in enforcing sanctions.

If North Korean goods can make their way into China, she wrote, “those consignments will probably successfully evade most screening by other states.”

Lax enforcement by Beijing could lead to increased conflict with Washington.

New unilateral U.S. sanctions imposed on North Korea require the U.S. Department of Homeland Security (DHS) to report on foreign ports that do not adequately screen North Korean cargo.

FILE - A Chinese-built fence near a concrete marker depicting the North Korean and Chinese national flags with the words "China North Korea Border" at a crossing in the Chinese border town of Tumen in eastern China's Jilin province.

Beijing’s support for sanctions is aimed at pressuring its traditional ally to halt its nuclear program in exchange for economic assistance and security guarantees.

But China also wants to maintain regional stability. So how flexible it will be in allowing coal imports and other trade for humanitarian reasons also remains to be seen.

“China will not completely draw its sword unless it is prepared to finish [its opponent.] It always leaves some room [to maneuver,]” said Professor Woo Su-keun, a Korea analyst at Donghua University in Shanghai.

Russia pressed for a revision to an earlier draft of the text banning the sale of aviation fuel, which is also used to power rockets, to North Korea. The final resolution includes an exception for civilian passenger aircrafts.

A North Korean mining executive with ties to Russia was also removed from a list of individuals designated for asset freezes and travel bans at Moscow’s urging.

Positive assessments

Joshua Stanton, an analyst with One Free Korea, a long time advocate for increased North Korean sanctions and a critic of past measures and enforcement efforts, called the new U.N. sanctions, “strong text – very strong.”

On his blog he wrote the financial sanctions have exceeded his expectations and “will effectively sever much of North Korea’s access to the global financial system.”

The U.S. Treasury Thursday added Hwang Pyong So, vice chairman of the North's powerful National Defense Commission, to its list of 16 other officials and individuals who are now subject to a travel ban and asset freeze. The measures are also designed to block U.S. nationals from dealing with them.

Hwang, who holds the rank of Vice Marshal in the North Korean army, heads its General Political Bureau, often seen as the most powerful position in the military after Kim Jong Un, who is the supreme commander.

VOA's Pam Dockins at the State Department and Youmi Kim and Han Sang-mi in Seoul contributed to this report.

WATCH: US Ambassador to UN Samantha Power on sanctions

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